Optical transmission fibres with optical amplification, comprising a core and a cladding, wherein the core material is doped with a first dopant of a material having a higher index of refraction than the material of the cladding and with a second dopant of a material that converts pump energy in the form of light having a first wavelength into light having a second wavelength different from the first wavelength for amplifying an optical signal beam to be transmitted by the transmission fibre, are known from US patent publication U.S. Pat. No. 6,467,313.
The core material of the known transmission fibres contains a third dopant besides the first and the second dopant. These dopants are all individually and uniformly distributed over the core material. The third dopant has been added for the purpose of limiting the variation in the amplification characteristics of the transmission fibre in the wavelength range between 1525 and 1575 nm.
The technical developments and future requirements in the field of optical telecommunication must be taken into serious account when planning and constructing new optical fibre networks. New optical fibre connections must be suitable for future use in several wavelength ranges so as to enable the use of wavelength multiplex systems and higher transmission rates. As a result, the requirements to be made as regards the attenuation, the sensitivity to nonlinear behaviour and the dispersion are much more stringent than the requirements that suffice for meeting the short-term transmission needs.
To meet the long-term needs, requirements are made not only as regards the passive performance of transmission fibres in transmitting signal beams but also as regards the performance of transmission fibres for possible amplification of the optical signal beam by the fibre itself.
Optical amplification of signal beams in optical fibres can be effected inter alia by means of the so-called Raman effect, or by means of stimulated emission.
When the Raman effect is used, energy in the form of an optical pump signal having a wavelength different from the wavelength of the signal beam is supplied to the core of an optical fibre together with the signal beam that is to be amplified. By selecting the wavelength of the pump signal so that the wavelength that is shifted relative to the wavelength of the pump signal, at which the amplification to be generated by the Raman effect reaches its peak value, coincides with the wavelength of the signal beam to be amplified, signal beams having a wavelength range of about 20-30 nm can be amplified. By using several pump signals having suitably selected wavelengths in relation to each other, signal beams having a wavelength within a large wavelength range can be amplified in this manner.
Raman amplification is used, inter alia, in optical amplifiers in which an amplification fibre arranged for optical amplification is used. Such an amplification fibre is wound on a small coil, which forms one unit together with the components used for coupling light into and out of the fibre and a pump laser. Such amplifiers are used in communication systems as power amplifiers for the transmitter, as pre-amplifiers for the receiver and as repeaters in long connections, such as transoceanic connections.
The Raman effect can be applied by means of a pump signal in the direction of the optical signal beam to be transported (“co-directional pumping”) as well as in the opposite direction (“contra-directional pumping”).
In another application of the Raman effect, a signal beam is amplified by transmitting a pump signal from the transmitter in the direction of the receiver and a pump signal from the receiver in the direction of the transmitter, both of which pump signals are supplied to the core of the transmission fibre so as to amplify the signal beam distributed over the length of the transmission fibre therein.
The extent to which the Raman amplification is generated in an optical fibre is expressed by the Raman amplification factor. This factor depends on the material properties and on the profile of the index of refraction of the core, which determines the power distribution in a direction transversely to the direction of propagation both of the signal beam and of the pump signal. For a standard single mode fibre the Raman amplification factor is about 0.3 W−1·km−1, for shifted dispersion fibres it is about 0.8 W−1·km−1.
When stimulated emission is used, a pump signal in the form of light is supplied to the core of an optical fibre together with the signal beam to be amplified. The core material is doped with a material having an energy level wherein the electron population is inverted relative to a lower energy level by the pump signal so as to provide an emission of light to be stimulated by the signal beam for amplifying the signal beam.
Optical amplifier fibres are used for amplification through stimulated emission, wherein the core material contains a high concentration of the dopant that is active in the conversion of the light of the pump signal. Since standard transmission fibres as such are not arranged for amplifying a signal beam through stimulated emission, and consequently the core material of these transmission fibres is not doped with a material that is active in the conversion of the light of the pump energy and the related amplification of the signal beam, hardly any amplification resulting from stimulated emission is observed in standard transmission fibres.
On the other hand, amplification fibres are not suitable for use as transmission fibres for transmitting signal beams without the presence of pump signals, because the high concentration of the dopant in the core material of these optical fibres that is active in the conversion of light of the pump signal and the related amplification goes hand in hand with a high absorption of the light of the signal beam and a concomitant strong attenuation of the signal beam.